This invention relates to continuous casting of metals and pertains more particularly to methods and apparatus for the production of continuous lengths of steel bars which have such improved surface qualities that the bars are suitable for directly forming into a wrought product.
In the usual methods for the continuous casting of metals such as steel, the molten metal is poured into an open ended vertical mold. The mold chills the periphery of the metal and solidifies a skin or shell on the mold wall to define a strand which is withdrawn continuously from the bottom of the mold while molten metal is poured continuously into the top of the mold at a rate adjusted to equal the withdrawal rate. After issuing from the mold, the hot strand is cooled, for example, by water sprays directly on the semi-solid strand to form a fully solidified strand. The cooling applied to the strand after it issues from the mold is known in the art as secondary cooling and is sufficient to complete the solidification of the strand prior to any subsequent processing.
In most continuous casting installations, the axis of the mold is vertical and the strand issues vertically downward therefrom. After the strand is completely solidified, pieces of the desired length are severed from the moving strand. Because it is necessary that the strand be completely solidified before cutting, casting speeds have been limited by vertical height considerations. That is, it has been necessary to limit casting speeds in order to permit complete solidification to take place within reasonable vertical dimensions between the mold and the cutting station. Otherwise, plant construction costs become excessive.
In the casting of steel, these problems have been of particular concern because of the high temperature of the molten steel, and the long time required to completely solidify the strand. For example, in typical installations for the continuous casting of steel, a distance of seventy feet between the mold and the cutting station is not uncommon, and even this distance requires restriction of the casting speed to less than that which is theoretically possible.
In order to reduce the vertical height requirements, it has been proposed to cast the strand in a vertically disposed mold, then to cool the emerging strand in a vertically disposed secondary cooling zone in which the casting is supported by rollers. The strand is then bent toward the horizontal by pairs of pressure rollers. In such installations, the strand is bent through an arc of approximately 90.degree. so that the bent strand becomes tangent to the horizontal. At the tangent point, the strand is rebent and straightened by pairs of pressure rollers, and it is then transported horizontally to a cutting station. This permits some reduction of machine height, but has not provided a satisfactory solution of the problem because a bending arc of relatively long radius is required. Even with a large radius, there is still difficulty in bending and rebending the solidified casting without cracking or otherwise damaging the casting.
A further reduction of height and overall length of casting machines has been achieved by making the mold cavity curved so that the strand emerges from the mold in curved condition conforming to the curved path. Molds with curved cavities, however, have not been completely satisfactory. Mold cavities are customarily provided with liners of copper because of its good heat conducting properties. The curved copper mold liners have higher fabricating and maintenance costs than straight copper liners for straight mold cavities. In addition, proper aligning of a mold with a curved cavity is more difficult than properly aligning of a mold with a straight cavity. However, the strand which emerges in straight condition from a straight mold cavity must then be bent into the curved path and this bending operation requires additional vertical space as compared with the vertical space requirement for machines having curved mold cavities. Thus, in known casting machines the benefits of conducting the strand along a curved path from the mold warrant the continued use of curved paths, but these benefits have been diminished by the above described problems with the molds.
In addition to efforts to reduce the vertical space required for continuous casting there has been a continuing effort to increase the casting speed. It is known that continuous relative motion between the casting and the mold impedes the transfer of heat from the solidifying casting to the mold wall and thus limits the casting rate. To date the most notable increase has been achieved by oscillating the mold along a short path in the casting direction as disclosed by Junghans in U.S. Pat. No. 2,135,183 (U.S. Class 164-83). For casting steel a usual amount of oscillation of the mold is about 1/10 to 1/30 the length of the mold, 1/16 to 2 inches, for example. In known constructions, molds having curved mold cavities are oscillated in an arc corresponding to the curvature of the path along which the strand is conducted from the mold. If, however, a mold having a straight cavity is used--to avoid the above-mentioned difficulty with curved mold passages--the strand must be conducted from the mold in a straight vertical line for a sufficient distance to avoid rubbing of the lower edge of the mold against the portion of the casting at the inside of its arcuate path. But this involves increasing the vertical space required. In addition, tests have shown that at higher casting speeds a strand cast in a straight mold cavity and then bent to follow a curved path from the mold tends to develop internal defects and surface cracks.
A much more serious problem, common to both straight and curved mold cavities, is one which arises as a direct consequence of increased casting rate, namely, the problem of obtaining satisfactory surface characteristics.
A universal characteristic of castings produced by a Junghans-type oscillating mold is the presence of oscillation marks or rings extending around the casting in the surface thereof. Due to friction between the advancing cast bar and the oscillating mold surface, axial stresses are imposed on the thin solidifying metal shell. These alternating stresses are thought to cause the observed surface cracks or other defects at intervals along the length of the casting usually in the form of rings around the entire circumference of the strand. These rings are spaced at distances equal to the total advance of the casting between successive strokes of the mold. That is, if the total advance of the casting (usually moving continuously at a constant rate) is two inches between the beginning of one retracting stroke of the mold and the beginning of the next succeeding retracting stroke, the rings will be found to be spaced at two inch intervals. Further, the width of the rings, i.e., the distance lengthwise of the casting over which these defects may be observed, varies depending on the conditions of the casting operation. With extreme care and operating at a low casting rate, the effects may be minimized, but in general, the width of the rings is related to the time of the retracting stroke of the mold. That is, if the return stroke consumes one-fourth of the time of a complete cycle, the rings will be formed to cover at least one-fourth of the surface of the cavity.
These rings are characterized by a roughened exterior surface of the cast bar, frequently with surface cracking, and frequently with evidence of "bleeding" i.e., the leaking of molten metal through a lesion in the formerly modified skin of the casting, with subsequent solidification of the leaking metal. The crystalline structure of the metal lying just under the rings is also irregular and disturbed.
In the case of non-ferrous metals, these effects have been undesirable, but not too serious. In many cases, despite the surface imperfections the castings could be rolled, extruded or otherwise processed without difficulty. In other cases a light scalping or other surface conditioning operation was sufficient to remove all objectionable surface imperfections. In the case of steel, however, such surface imperfections cannot be tolerated, and it is not economically feasible to remove the imperfections by scalping. Moreover, the economics of the continuous casting of steel demands a far greater casting rate than is customary or desirable in casting non-ferrous metals, and it has been found that the increased casting rate greatly magnifies the difficulty. Thus, in casting non-ferrous metals on this type of mold a casting rate of thirty to sixty inches per minute is usually adequate, and at these speeds, the surface imperfections are tolerable in non-ferrous metals. In casting steel, on the other hand, casting rates as high as two hundred inches per minute have already been successfully achieved with the Junghans-type process, but this success is tempered by the fact that at about these speeds and at greater speeds, the surface imperfections within the ring areas are often extremely bad. Between successive rings, the surface is usually good and the interior crystalline structure is acceptable.
From the theoretical point of view, therefore, the ideal form of mold for continuous casting would be a curved one of greatly extended length, but since as a practical matter this cannot exist, other devices have been utilized.
Thus, it has been proposed to use endless supports such as revolving drums, wheels and the like, or endless moving bands or endless chains of mold sections which join together to form a mold at the start of the solidification process and separate at its conclusion to release the solidified metal. Since the surfaces of such movable supports can remain stationary with respect to the metal during the solidification process, favorable conditions are provided for the solidification of metal with good crystalline structure and smooth surface characteristics. But while such methods offer some theoretical advantages, actual experience with them has been disappointing. Constructional and operating difficulties have provided so many obstacles to practical successful operation that such methods have made little or no headway in actual commercial operation.
Therefore, on balance, for the continuous casting of steel the use of oscillating molds with curved cavities has, up to the present, been considered the most satisfactory arrangement for reducing the height of the apparatus and for increasing the rate of casting, despite the problems with oscillating curved mold liners, described above.
Horizontal molds have been utilized heretofore for the continuous casting of aluminum and some other non-ferrous metals in machines in which the molten metal is introduced into a horizontal mold through a refractory feed spout which extends through the end wall of the mold. When casting aluminum, the feed spout is not wet by the molten aluminum and it remains clean as casting proceeds. However, when casting steel, and in particular, where it is desired to use an oscillating mold, this type of horizontal mold with a refractory feed spout cannot be employed. It has been found that steel wets the spout and solidifies around the spout. The solidified steel tends to build up a false tube extending the length of the mold, ultimately resulting in a breakout of molten metal at the exit end of the mold.
In addition, it is known that the position and direction of the inflowing stream of molten metal greatly affects the solidification process and therefore the resulting product.
A horizontal casting mold usually necessitates a horizontal inflowing stream of molten metal which washes against metal which is already beginning to solidify on the mold wall. This causes the solidifying metal to remelt, often resulting in bleeding of molten metal to the outside of the casting. If the velocity of the inflowing metal is high or is such to cause turbulence in the pool of molten metal, bubbles of gas and particles of oxides, slag, or dirt floating on the surface of the molten metal may be entrapped, causing holes and inclusions in the casting, sometimes even resulting in gross porosity or "piping" in the casting. At the very least, a horizontally solidified bar exhibits internal variations across its section due to the effects of gravity. For example trapped gasses and light particles tend to float upwards toward the topside of the bar. Thus the center of the bar may be sound but an area of porosity or of inclusions is located near one edge of the bar. This off-center distribution of defects is often more serious than center defects since it causes unpredictable variations in subsequent processing, e.g., hot-rolling into rod. Consequently, it is desirable that the pool of molten metal be open or exposed at the top so that trapped gasses and other impurities can avoid being trapped into the solidifying bar, or at least confined to the center where they are least harmful.
When a continuous casting of rectangular cross section initially solidifies inside a typical horizontal mold, the (usually) larger top and bottom surfaces are necessarily exposed to more rapid cooling. The resulting shrinkage effects cause these surfaces, especially the top, to pull away from the walls of the mold before moving very far from the molten pool thus slowing the initially rapid cooling. Since the several edges and surfaces do not all shrink uniformly, the cooling rates and therefore temperatures, stresses, and thickness of the frozen shell all differ from one surface to another. These drawbacks become more pronounced at higher casting rates and as the casting continues to move through the mold, bright and dark areas appear on the slab as it issues from the mold. The bright areas often indicate high temperature locations where remelting of the once frozen shell can occur. Remelting occurs due to the transfer of heat from the still hot interior of the bar. At these points of weakness, the stresses in the frozen shell produce cracks which can cause breakouts or other surface defects.
Moreover, the unequal stresses have another undesirable consequence, namely that of causing a type of geometrical distortion of the cast bar known as rhombic distortion which is a nuisance in subsequent processing of the casting.